1. Field of the Invention
The present invention relates to a compound lens in which three or more optical elements are cemented together and an optical system including the same. The present invention can suitably be applied to optical apparatuses such as film cameras, digital still cameras, video cameras, telescopes, binoculars, projectors, and digital copying machines.
2. Description of the Related Art
Optical systems (image taking optical systems) for use in optical apparatuses such as digital cameras and video cameras developed in recent years are required to have high performance and to be small in overall size and light in overall weight. Generally speaking, a reduction in the size of an optical system tends to lead to larger aberrations, in particular larger chromatic aberrations including longitudinal chromatic aberration and lateral chromatic aberration, whereby optical performance is deteriorated. It is difficult to achieve high performance and reduction in size and weight at the same time in optical systems using only existing optical materials or glasses.
Known methods of solving the above-described problem include correcting aberrations using a compound aspherical lens and correcting aberrations using a compound lens in which an organic-inorganic composite material is used (see Japanese Patent Application Laid-Open No. H05-337959 and Japanese Patent Application Laid-Open No. 2007-163964).
Japanese Patent Application Laid-Open No. H05-337959 discloses a compound aspherical lens produced by forming an aspheric surface by molding an organic composite material on a surface of a spherical lens made of a glass material.
Japanese Patent Application Laid-Open NO. 2007-163964 discloses a compound lens using an organic composite material having anomalous partial dispersion.
The optical element disclosed in Japanese Patent Application Laid-Open No. H05-337959 and Japanese Patent Application Laid-Open No. 2007-163964 does not necessarily have satisfactory heat resistance or environmental resistance as compared to optical elements made of only inorganic glass materials.
More specifically, in compound aspherical lenses in which a synthetic resin layer is provided by molding to make a spherical lens surface aspherical, since the synthetic resin layer is exposed to the atmosphere, the surface shape and optical characteristics tends to change with environmental changes such as changes in the temperature and humidity. Furthermore, if the hardness of the synthetic resin layer is low, it can easily be scratched, and/or there is a risk of cracking. Although compound lenses can easily designed to have no optically effective surface of an organic composite material that is exposed to the atmosphere, it is apt to be affected by humidity and temperature through its edge that is exposed to the atmosphere. To improve the environmental resistance, it is effective to make the area of the edge that is exposed to the atmosphere smaller.
However, the optical element disclosed in Japanese Patent Application Laid-Open No. H05-337959 and Japanese Patent Application Laid-Open No. 2007-163964 does not necessarily have a design excellent enough to improve the environmental resistance.